1. Field of the Invention
The present invention relates to a liquid crystal display device having at least one, preferably a plurality, of pixel elements.
2. Description of the Prior Art
In standard liquid crystal display device, the pixel element is a liquid crystal layer (normally common to the pixel elements) extending in a plane, and there is at least one polarizing means parallel to the plane of the liquid crystal layer. By applying electrical signals to the liquid crystal layer using suitable electrodes, it is possible to vary the angle of polarization of polarized light passing through the liquid crystal layer. Thus, by changing those electrical signals, it is possible to vary the optical transmissivity of a liquid crystal display device by varying the change in polarization relative to the at least one polarizing means. Normally, in such a liquid crystal display device, the polarizing means is formed by two polarizing plates, one on each side of the liquid crystal layer, but is also possible to provide an arrangement with a single polarizing plate on one side of the liquid crystal layer, and a reflective element on the other side of the liquid crystal element.
In standard liquid crystal display devices, electrical fields are generated by the electrodes perpendicular to the plane of the liquid crystal layer. Therefore, if the change in the liquid crystal layer due to the electric fields is to be visible, the extent of those electrodes needs to be large, and therefore it is necessary to use transparent electrodes. Furthermore, at least two layers are normally needed between the transparent electrodes on each side of the liquid crystal layer and the liquid crystal layer itself. One layer forms an orientation layer for the liquid crystal layer, but a further insulating layer is then needed between the orientation layer and the transparent electrode.
In International Patent Application No. PCT WO91/10936, a liquid crystal display device was disclosed in which electrical signals were applied to the liquid crystal layer so as to generate electric fields having components in a direction parallel to the plane of the liquid crystal layer. Such parallel field components cause reorientation of the molecules of the liquid crystal layer, thereby varying the optical transmissivity of the liquid crystal display device.
In PCT WO91/10936, it was proposed that the electrodes for applying such field were, for each pixel element, in the form of combs, the teeth of the comb formed by one electrode extending into the spaces between the teeth of the comb formed by the other electrode. The teeth of each electrode were electrically connected in common, and a voltage was applied between the electrodes.
JP-B-63-21907(1988) also disclosed a liquid crystal display device in which electrical signals were applied to the liquid crystal layer so as to generate electric fields having components in a direction parallel to the plane of the liquid crystal layer. As in PCT WO91/10936, the electrodes for applying such fields were, for each pixel element, in the form of combs. Use of comb-shaped electrodes was also disclosed in U.S. Pat. No. 4,345,249.
In each of these known arrangements, each pixel element thus has first and second electrodes of comb shape, with the teeth of one comb extending between the teeth of the other comb. Voltages are then applied to the electrodes by a suitable control circuit. It is important to note that the teeth of the comb-shaped electrodes are not electrically independent, so that size of the pixel is determined by the size of the comb-shaped electrode.
The principles of operation of such devices, with comb shaped electrodes, is also discussed in an article entitled "Field Effects In Nematic Liquid Crystals Obtained With Interdigital Electrodes by R. A. Soref in the Journal of Applied Physics, pages 5466 to 5468, vol. 45, no. 12 (December 1974), and in article entitled "Interdigital Twisted-Nematic-Displays" by R. A. Soref published in the Proceedings of the IEEE, pages 1710 to 1711 (December 1974).